1. Field of Invention
The disclosure herein relates to a process of chemical transformation employing means to form polyamides and a process for producing the same from dicarboxylic acids and diamines, nylon salts or lactams; and combinations of these. More particularly this invention relates to a process for forming polyamides, including those polyamides having high melting temperatures. The process disclosed herein provides for the suppression of thermal degradation, rearrangement and/or side reactions which may occur at high temperatures, or at higher processing temperatures demanded by a higher melting temperature polyamide. In particular the polymerization process herein relates to one conducted in an ionic liquid (IL).
2. Description of the Related Art
There are known methods by which polyamides may be synthesised. Weber in Kirk-Othmer Encyclopedia of Chemical technology, Fourth Edition, Volume 19, page 454-518; and Zimmerman in Encyclopedia of Polymer Science and Technology, Second Edition, John Wiley, Volume 11 pages 315-381, and Preston in Encyclopedia of Polymer Science and Technology, Second Edition, John Wiley, Volume 11 pages 381-409 and references therein, describe such methods.
Where a reactant used to form a polyamide polymer or the polymer itself is sensitive to temperatures below the polymer melt temperature, the “Acid Chloride reaction” is often used to produce the polymer. Typically, the reaction is carried out between a diacid chloride and a diamine in a solvent for the reactants at low to moderate temperatures in the presence of a base to neutralise the hydrogen chloride produced. If the polyamide is insoluble in the solvent used for the reaction and precipitates out as it forms, this may limit the achievable molecular weight. To overcome this molecular weight limitation, powerful solvents are employed, often based upon amides such as dimethylacetamide with added calcium salt. Such a method, and variants of it, are used commercially to produce wholly aromatic polyamides, commonly known as aramides. However, acid chlorides are expensive raw materials requiring special handling due to their corrosive nature and their sensitivity to moisture.
It would be more desirable to use readily available, easily handled, low-cost raw materials, such as organic dicarboxylic acids and organic diamines; nylon salts; or lactams. However, these substances tend to be much less reactive than the acid chlorides. Other means to induce a direct polymerization reaction using such materials is of ongoing interest in the art.
The “direct polycondensation” of aromatic diamines with aromatic dicarboxylic acids may be accomplished in solution, often amide solvents with added lithium or calcium salts to keep the products in solution during the whole reaction, by the addition of triarylphosphite, such as triphenylphosphite, and sometimes in the presence of species such as pyridine to activate the reaction. In this type of reaction the water formed as the condensate of the polycondensation reaction is effectively removed from the reaction by reacting with, and hydrolysing the triphenylphosphite, and hence drives the reaction to completion. A significant disadvantage of this process is that the hydrolysis results in the liberation of phenol, which must be handled with care and in commercial practice the phenol is isolated and treated as a co-product.
Another means of accessing the desirable monomer feed stocks is to use high temperatures to activate and to induce the polymerization reaction. In the case of lactam monomers additional species, such as water and nylon salts, are used to initiate the ring opening reaction.
Today in commercial processes for producing aliphatic or semi-aromatic polyamides a melt polymerization reaction, either the “direct amidation” method (organic dicarboxylic acids and organic diamines; or nylon salts, or amino-alkanoic acids) or the hydrolytic “ring opening” reaction (lactams) are used. In these types of reactions high temperature and pressure profiles are employed to maintain the polymerizing material in a fluid state with the increasing level of polymerization. Nylon 6,6 and Nylon 6 are typical of aliphatic polyamides made by a melt polymerization method; both of these polymers are complete in their polymerization process at around 285° C. At the finish of the polymerization process, the polymer is pelletized ready for further processing, such as fibre spinning, compounding or further polymerization in the solid state.
Some polyamides, however, are less amenable to being produced by melt polymerization methods. Nylon 4,6 polymer, for instance, has a higher melting point than Nylon 6,6 and so any melt polymerization method must be completed at even higher temperatures than that used for Nylon 6,6 or Nylon 6. However, a significant drawback of this is that thermal degradation reactions occur at these temperatures, producing species that limit the molecular weight of the polymer. European Patent No. 039524 discloses a method to produce a polymer of useful molecular weight where the polymerization reaction is terminated whilst the polymer is still at a low molecular weight, effectively forming a “pre-polymer” with a content of molecular weight-limiting species that still allows some further polymerization by lower temperature, 260° C., solid phase polymerization methods. European Patent No. 077106 discloses a process for making Nylon 4,6 by heating adipic acid and 1,4-diaminobutane in an inert polar organic solvent, such as 2-pyrrolidone, at temperatures above 150° C., up to 180° C. as exemplified, and in which the polymer dissolves at these temperatures. That the temperature of reaction in the exemplification is much higher than the flash point of the 2-pyrrolidone solvent (113° C.) makes this an unattractive commercial process because of the challenging processing conditions.
Another family of polyamides less amenable to being produced by melt polymerization methods are those which contain a significant amount of aromatic dicarboxylic acids, such as terephthalic acid or isophthalic acid. Such monomers may confer some desirable properties to the polyamide, such as higher glass transition temperature or high melting point or higher viscosity for a given degree of polymerization as compared to all aliphatic polyamides. But they also bring with them disadvantages, such as higher melt viscosities which cause problems when extruding the polymer from the melt polymerization vessel; those with a high melting point have to be processed at temperatures where significant side reactions occur and give rise to branched species which may be deleterious to physical properties in the final product.
In the art of polyamide polymer formation there remains a need for polymerization methods conducted at lower temperature and useful with aliphatic and aromatic content polymers using readily available, easily handled, low cost raw materials. Such temperatures should be lower than those demanded by the traditional polyamide melt polymerization methods. The process chemistry should also be achievable without the generation of low-value co-products or difficult processing conditions.